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Geotechnical Engineering Evaluation Lyashenko Accessory Dwelling Unit Development Everett, Washington NGA File No. 1165620 April 10, 2020 Page 6 Site Preparation and Grading After erosion control measures are implemented, site preparation should consist of stripping the topsoil, undocumented fill and loose soils from foundation, slab, pavement areas, and other structural areas, to expose medium dense or better native bearing glacial soils. The stripped soil should be removed from the site or stockpiled for later use as a landscaping fill. It may not be feasible or economical to remove all of the fill materials from proposed pavement areas. At a minimum, to avoid significant pavement settlement we recommend a minimum of 12-inches of over excavation and replacement with crushed rock structural fill. Based on our observations, we anticipate stripping depths of 4.0 to 7.5 feet, depending on the specific locations. However, additional stripping may be required if areas of deeper undocumented fill and/or loose soil are encountered in unexplored areas of the site. Alternatively, if deep foundations systems are to be utilized within the site, preparation would consist of stripping topsoil and grading foundation areas down to desired subgrade elevation and then installing piles. After site stripping, if the exposed subgrade is deemed loose, it should be compacted to a non -yielding condition and then proof -rolled with a heavy rubber -tired piece of equipment. Areas observed to pump or weave during the proof -roll test should be reworked to structural fill specifications or over -excavated and replaced with properly compacted structural fill or rock spalls. If loose soils are encountered in the pavement areas, the loose soils should be removed and replaced with rock spalls or granular structural fill. If significant surface water flow is encountered during construction, this flow should be diverted around areas to be developed, and the exposed subgrades should be maintained in a semi -dry condition. If wet conditions are encountered, alternative site stripping and grading techniques might be necessary. These could include using large excavators equipped with wide tracks and a smooth bucket to complete site grading and covering exposed subgrade with a layer of crushed rock for protection. If wet conditions are encountered or construction is attempted in wet weather, the subgrade should not be compacted as this could cause further subgrade disturbance. In wet conditions it may be necessary to cover the exposed subgrade with a layer of crushed rock as soon as it is exposed to protect the moisture sensitive soils from disturbance by machine or foot traffic during construction. The prepared subgrade should be protected from construction traffic and surface water should be diverted around areas of prepared subgrade. NELSON GEOTECHNICAL ASSOCIATES, INC.